The present invention relates to a two-stroke cycle diesel engine, and, more particularly, to a two-stroke cycle diesel engine adapted for use as a small-size automobile engine.
A two-stroke cycle engine has theoretically the advantage that an engine of a certain size can generate a greater power than a four-stroke cycle engine of an equivalent size because the two-stroke cycle engine has twice as many work cycles per revolution as the four-stroke cycle engine. However, with regard to conventional small-size diesel engines, in practice, power output per unit effective stroke volume of a two-stroke cycle engine is not substantially different from that of a four-stroke cycle engine. This is due to the fact that scavenging is insufficient in small-size two-stroke cycle diesel engines and power output per unit effective volume per combustion stroke is very low. In fact, a volumetric efficiency as high as 80% is available in four-stroke cycle engines, while on the other hand the volumetric efficiency of the typical two-stroke cycle engine is still as low as 40-50%. Conventional small-size two-stroke cycle diesel engines mostly depend upon crankcase compression for the compression of scavenging air. However, since the pump stroke volume of crankcase compression is equal to the stroke volume of the engine, and since the crankcase has a relatively large clearance volume, the compression ratio of crankcase compression is relatively low, so that as a result the amount of air drawn into the crankcase is small, the amount of delivered air is small, the delivery pressure is low and hence the scavenging pressure is low. Consequently it is hard to supply a really adequate amount of scavenging air into the power cylinder. As a result, the delivery ratio obtained in an engine wherein scavenging is effected only by the normal crankcase compression is only as high as 0.5-0.8. Since the trapping efficiency is about 0.7, the volumetric efficiency becomes as low as 40-50%, as mentioned above.
The purpose of scavenging is to push the residual exhaust gases in the power cylinder out of it by fresh air. If the pressure of the residual exhaust gases and the distance between the scavenging port and the exhaust port are given, the time required for completing scavenging is determined by the pressure and the amount of scavenging mixture or air, provided that stratified scavenging is performed, wherein the amount of scavenging mixture or air determines how strongly the initial supply of scavenging mixture or air is backed up. Now, if the scavenging pressure is low, as when crankcase compression is used, a relatively long time is required for completing scavenging, particularly when the scavenging is performed by uniflow scavenging. When the engine is rotating at high speed, therefore, it may well occur that the exhaust port is closed before the scavenging is completed, so that a large amount of exhaust gases still remains in the power cylinder. In such situation only a very small amount of fresh air is charged into the power cylinder. Therefore, conventional two-stroke cycle engines have been unable to operate satisfactorily in the high-speed range.
Furthermore, when scavenging depends only upon crankcase compression, since a power piston also operates as a pump piston, as a matter of course, the operational phase difference between a power cylinder-piston assembly and a pump cylinder-piston assembly is exactly 180.degree.. Therefore, the pump piston of a pump cylinder-piston assembly just reaches its top dead center (TDC) when the power piston of a power cylinder-piston assembly reaches its bottom dead center (BDC). In this connection, in the present description, the TDC of a piston means the dead center of the piston at the end of the compression stroke of the piston, while the BDC of a piston means the dead center of the piston at the end of the suction or expansion stroke of the piston. In this case, however, although a half of the scavenging period is still left in the power cylinder-piston assembly when its power piston has reached its BDC, the pump piston now begins to move towards its BDC, whereby the pressure in the crankcase rapidly lowers after the power piston traverses its BDC, so far as to generate a partial vacuum in the crankcase, thereby causing the problem that the scavenging period is not all effectively utilized.
In view of the aforementioned problem of poor engine performance due to insufficient scavenging as mentioned above, we have proposed, in U.S. Pat. No. 4,185,596 and pending U.S. patent application Ser. No. 917,244, to improve the performance of two-stroke cycle gasoline engines by assisting or by replacing the conventional scavenging dependent only upon crankcase compression by scavenging dependent upon a reciprocating type pump cylinder-piston assembly which is separate from and is driven by the power cylinder-piston assembly in synchronization therewith. This substantially increases the pressure and the amount of scavenging mixture when compared with the conventional scavenging dependent only upon crankcase compression. By combining such a concept of scavenging by substantially increased pressure and amount of scavenging mixture with a power cylinder-piston assembly incorporating uniflow scavenging, volumetric efficiency of the power cylinder-piston assembly is substantially increased. To improve further the performance of such a two-stroke cycle gasoline engine, shifting the operational phase of the separate pump cylinder-piston assembly relative to that of the power cylinder-piston assembly so that the top dead center of the pump cylinder-piston assembly is behind the bottom dead center of the power cylinder-piston assembly by a certain phase angle provides an extension of scavenging period after the BDC of the power cylinder-piston assembly, so that the volumetric efficiency of scavenging of the power cylinder-piston assembly is further increased.
Further, we have proposed, in a pending U.S. patent application Ser. No. 966,597, a two-stroke cycle diesel engine employing a pump cylinder-piston assembly which separate from and is driven by the power cylinder-piston assembly in synchronization therewith, in addition to or instead of the conventional crankcase compression, for substantially increasing the pressure and the amount of scavenging mixture when compared with the conventional scavenging dependent only upon crankcase compression. Supplying scavenging air into the power cylinder in a relatively slowly swirling condition so that as highly stratified a condition as possible should be maintained in the power cylinder suppresses mixing between the exhaust gases and the scavenging air during the primary scavenging process of expelling exhaust gases out of the power cylinder. Then, by generating strong swirl flow in the power cylinder depending upon high pressure and large amount of scavenging air, the danger of substantially lowering the trapping efficiency due to mixing between the exhaust gases and the scavenging air is eliminated, thereby shortening delay of ignition of fuel and increasing combustion speed of fuel. The engine, therefore, operates at high volumetric efficiency with high power output without causing diesel knock or smoking.
The abovementioned particular combination of scavenging by substantially increased pressure and amount of scavenging mixture available from a separate pump cylinder-piston assembly and the uniflow scavenging of a power cylinder-piston assembly depends upon the idea, confirmed by experiments, that it is possible to push the exhaust gases existing in the power cylinder uniformly out of it by the scavenging mixture at high pressure without causing any detrimental mixing between the scavenging mixture and the exhaust gases if uniflow scavenging is employed. In this case, if the amount of scavenging mixture is increased so as to be necessary and sufficient, and if the duration of scavenging is long enough, scavenging at high scavenging efficiency is accomplished, and, as a result, the volumetric efficiency increases, resulting in corresponding increase of engine output power. By contrast, if cross or loop scavenging is used, the flow of scavenging mixture at increased pressure is liable to penetrate through the layer of exhaust gases existing in the power cylinder in a short-cutting manner, and scavenging mixture and exhaust gases may be mixed with each other, not only causing poor scavenging but also increasing blow-out loss of mixture, and lowering the volumetric efficiency.
Further, in the applications it has been proposed that the two-stroke cycle gasoline engine incorporating uniflow scavenging should have a two-stroke cycle power cylinder-piston assembly having two horizontally opposed pistons. This is related with the fact that the engine is particularly intended for use with automobiles. As described in detail in the specifications of the aforementioned former applications, currently there exists a great demand for the development of cars which have low fuel consumption, in view of energy saving. Cars also must satisfy a high standard with regard to the prevention of air pollution. In order to improve fuel consumption, not only improvement of the fuel consumption of the engine itself but also reduction of the air resistance of the vehicle are required.
We have noted, in connection with various running tests carried out to prepare for the qualification tests for conforming to the standard for the prevention of air pollution (which are becoming more severe), that fuel consumption is different in summer and in winter due to the difference of atmospheric air density, and we more keenly recognized that the air resistance of the vehicle has an important effect on the fuel consumption of the vehicle even in low speed running. In order to lower the air resistance of the vehicle, it is important to reduce the height of the vehicle as much as possible and to form the vehicle in a streamlined external shape. Particularly, it is very effective to lower the engine hood. In order to reduce the height of the vehicle it is effective to eliminate the driving shaft for driving the rear wheels so that the shaft tunnel is eliminated and the entire floor may be flat, thereby constructing a vehicle body having a low floor and a low roof. A method for accomplishing this is to employ the FF system, i.e. the front engine-front drive system. In order to lower the engine hood by a large amount in an automobile of the FF type while ensuring necessary legroom for the driver and the front seat passenger, it is necessary to reduce substantially the height and length of the entire engine compartment. Furthermore, in order to reduce the air resistance of the vehicle, it goes without saying that the frontal area of the vehicle must be reduced. Therefore the width of the vehicle should be minimized. Furthermore, since the transmission, differential gears, and other driving mechanisms must be housed in the engine compartment together with the engine, in the FF system, the space allowed for the engine is much reduced. Light trucks are often designed with the engine mounted under the driver's seat, and in such a design the engine, being relatively long, often extends so far backwards as to make a hump due to the engine enclosure rearward of the cabin, thus shortening the deck. Depending upon the recognition of these facts, the desire for obtaining an engine which is low in its height, short in its length, and yet not very large in its width, and which has relatively high output power when compared with its volume was combined with the idea of substantially increasing pressure and volume of scavenging mixture or air in a two-stroke engine incorporating uniflow scavenging so as to increase substantially the performance of the two-stroke cycle engine, resulting in the choice of a two-stroke cycle power piston assembly having two horizontally opposed pistons and incorporating uniflow scavenging.
In the aforementioned pending U.S. patent application Ser. No. 966,597, it is intended, by using a pump cylinder-piston assembly separate from a power cylinder-piston assembly, to increase substantially the volumetric efficiency in scavenging of a two-stroke cycle diesel engine. This is accomplished by not only increasing the power output of the engine, but also generating strong swirl flow of scavenging air directly in the power cylinder without causing reduction of scavenging efficiency due to mixing of the scavenging air and the exhaust gases, thereby expediting ignition and combustion of fuel so as to avoid the occurrence of diesel knock and/or smoking. With regard to the concept of giving strong swirl to the air introduced into the power cylinder, it is conventionally known to provide a vortex chamber at a part of the engine cylinder of a small-size high speed diesel engine for automobiles so as to produce strong swirl flow in the vortex chamber by the flow of air which flows from the main part of the engine cylinder into the vortex chamber. The fuel is injected into the vortex chamber. In this vortex chamber type engine stronger swirl flow is obtained in the vortex chamber as the rotational speed of the engine increases, with the air being more strongly driven into the vortex chamber. However, in this type of engine having a vortex chamber, or, in general, in an engine having an auxiliary combustion chamber, there is a drawback in that loss of effective energy due to throttling loss and vortex loss as well as heat loss due to increase of the cooling surface of the combustion chamber caused by an auxiliary chamber are relatively large, thereby resulting in increase of fuel consumption and reduction of the mean effective pressure. Therefore, it is intended in the present invention, as was so intended in the former proposition, to employ an engine of a direct fuel injection type which does not employ any auxiliary chamber, and, instead of this, to increase substantially the volumetric efficiency in scavenging as well as to generate strong swirl flow of scavenging air required for proper combustion of fuel. In this connection, it is relatively simple to generate a strong swirl flow of air in the power cylinder, without employing any vortex chamber, by employing a pump cylinder-piston assembly separate from a power cylinder-piston assembly so that the pressure and the amount of scavenging air are substantially increased when compared with scavenging only by conventional crankcase compression and by incorporating a proper air deflecting structure in the scavenging port. However, if such a strong swirl is given to the scavenging air from the beginning of its introduction into the power cylinder, great mixing between the scavenging air and the exhaust gases in the power cylinder is caused, even though there is no danger of causing the blow-out loss of fuel as in the case of a gasoline engine since only air is used for scavenging in a diesel engine. Thus there occurs the problem that the trapping efficiency is substantially lowered. In view of this problem, in the aforementioned pending U.S. patent application Ser. No. 966,597 , we have proposed a special device which is to supply scavenging air as a moderately swirling flow in the initial stages of scavenging so as to avoid mixing of the scavenging air and the exhaust gases. Then, after the scavenging has proceeded so far as to eliminate the danger of causing a substantial reduction of the trapping efficiency due to mixing between the scavenging air and the exhaust gases, scavenging air with strong swirl is supplied, so as finally to generate a strong swirl flow of scavenging air in the power cylinder which is required for desirable ignition and combustion of fuel. Although this swirl flow of scavenging air is somewhat attenuated during the subsequent compression stroke, a substantial part of the swirl still remains after the completion of the compression stroke, supplying fresh air around atomized particles of fuel, thereby expediting ignition and combustion of the fuel. When this particular scavenging structure is combined with the scavenging by high pressure and large amount of scavenging air available from a pump cylinder-piston assembly separate from the power cylinder-piston assembly, the power cylinder-piston assembly is scavenged at high volumetric efficiency and in such a way that the scavenging air is given stronger swirl as the rotational speed of the engine increases. Such a swirl of scavenging air is effectively preserved until the final stage of compression so that the fuel injected directly into the power cylinder is quickly and desirably dispersed in the swirl of air thereby effecting quick ignition and combustion of fuel and enabling the diesel engine to operate at high speed.
However, it is to be noted that the abovementioned high speed operation with regard to the two-stroke cycle diesel engine of the aforementioned prior application is located in a lower speed region than the high rotational speed region of conventional automobile four-stroke cycle gasoline or diesel engines, and is about 3800 rpm at the highest. This aims at reduction of internal friction losses in the engine and at increase of effective output power of the engine. Conventionally, a relatively small-sized four-stroke cycle engine for automobiles is designed so as to be operated at relatively high rotational speed so that relatively high power output is available from a relatively small-sized engine. In this connection, it is noted that, for example, in the case of an engine which has a two-liter piston displacement and produces 92 PS of brake horsepower at 5000 rpm, a very large proportion of the power, such as 52 PS out of the indicated horsepower of 144 PS, is consumed by internal friction losses in the engine. The ratio of the internal friction losses to the output power of the engine is substantially reduced by lowering the rotational speed of the engine. In view of this, and in view of the fact that a two-stroke cycle engine can generate higher power than a four-stroke cycle engine at lower rotational speed if its volumetric efficiency is increased, since the feature that a two-stroke cycle engine has twice as many work cycles per revolution as a four-stroke cycle engine, the invention of the aforementioned prior application contemplates effective utilization of this feature by increasing the volumetric efficiency of a power cylinder. This is accomplished by the combination of scavenging by high pressure and volume of scavenging air and uniflow scavenging, and, to increase power output, by giving high swirl to scavenging air in accordance with the rotational speed of the engine. Thus, the rotational speed of the engine may be lower than conventional small-size four-stroke cycle automobile engines, while the net output power per unit stroke volume of the engine may be increased. In this connection, from the point of view of effecting good scavenging of the area next to the piston head, it is desirable that the piston head should have a nearly planar surface. However, from another point of view of obtaining a squashing effect of combustion air in the final stage of compression stroke, it is also desirable that a cavity should be formed in the piston head so as to add a swirl flow around an axis perpendicular to the axis of the main swirl flow which rotates around the central axis of the power cylinder, thereby generating complex swirls which further expedite ignition and combustion of fuel.
On the other hand, it has been the problem for years in diesel engines to suppress diesel knock and smoking, and this problem is even more serious when a diesel engine is employed as an automobile engine. In the aforementioned prior application with regard to a diesel engine, we contemplated solving this problem by giving strong swirl, in a particular manner, to the scavenging air introduced into the power cylinder.
Rudolf Diesel, the inventor of the diesel engine, states in the specification of his basic patent that in the diesel engine fuel must be supplied gradually over a certain time period. However, in accordance with modern research with regard to diesel engines, it has been found to be effective for the suppression of diesel knock and smoking, supplying a small amount of fuel as a pilot charge, and then, after the lapse of a distinct time interval, supplying a main charge of fuel, rather than supplying fuel gradually, is more effective. This is due to the fact that when a pilot charge of fuel is supplied in advance of the supply of the main charge of fuel by a certain time, the fuel of the pilot charge is already in combustion at the time the main charge of fuel is supplied. This avoids abrupt combustion of all the fuel together at one time after the lapse of a combustion delay time, which is diesel knock in conventional diesel engines. Up to now this pilot charge method has been considered to be most effective for suppressing diesel knock and smoking. However, this method requires provision of a first fuel injection system which supplies a predetermined amount of pilot charge of fuel regardless of engine load and a second fuel injection system which supplies a main charge of fuel the amount of which varies in accordance with engine load.
Another method of suppressing diesel knock and smoking is known "fumigation". In this method, a small amount of fuel is sprayed into scavenging air which does not unergo ignition by compression, i.e. pre-ignition, of the fuel/air mixture during compression stroke. The fuel thus pre-sprayed into the scavenging air is decomposed, without igniting, by heat applied during compression stroke, so as to generate chemically active radicals such as C2, CH, CHO, OOH, H, OH, which expedite initiation of combustion of fuel. However, in order to obtain good results from this fumigation with regard to suppression of diesel knock and smoking, the fuel sprayed into the air introduced into the power cylinder during suction stroke must have the form of very fine particles, or the fuel should be vaporised. According to all test results available to us, in order to obtain good results from fumigation of spraying type, it is required that the sprayed fuel particles should have mean diameter of about 4 microns. Therefore, a diesel engine which employs this conventional fumigation requires a fine fuel particle generating means, which is generally a high pressure apparatus, for convenient fuel spraying, as well as a large scale heating means for heating air including fuel particles (this is called fume) so as to vaporize these fuel particles before the air is introduced into the power cylinder. If a convenient fuel spraying means such as a conventional carburetor or fuel injection means alone is used without a fume heating means, the fuel particles will be too large to suppress effectively diesel knocking or smoking.